The present invention relates generally to the technology of induction heating and more particularly to the use of induction heating for case-hardening of machine components such as gears.
Machine components such as gears, splined shafts and sprockets are frequenly subjected to high torque loads, frictional wear and impact loading. The gears in a power transmission, for example, will encounter each of these forces during normal operation. In the typical gear production facility, the machining of gear teeth is followed by heat treatment to harden them. Heat-treating gears can involve many different types of operations, all of which have the common purpose or singular objective of producing a microstructure with certain optimum properties. The hardening process, however, often distorts the gear teeth resulting in reduced and variable quality.
In order to avoid these problems associated with conventional heat-treating and to improve the ability of the machine component (gear) to withstand the aforementioned loads and wear forces, the base metal is given a hardened outer case by selective hardening. In this manner, it is only the outer surface which is altered and the base metal retains its desirable properties such as strength and ductility.
One technique for the selective hardening of this outer case on such machine components as gears is to induction-harden the gear teeth individually. Another hardening technique which is also selective is a process referred to as selective carburizing. Single-tooth induction hardening is performed with a shaped intensifier that oscillates back and forth in the gear tooth space. This is usually done with the gear submerged in the quench. The process is relatively slow because only one gear tooth is processed at a time. Selective carburizing is most widely used and the process involves covering the surfaces to be protected against carburizing with a material that prevents the passage of active carbon during the furnace operation. The most widely used method to stop carbon activity is copper plating. A gear is copper plated on all surfaces except the teeth, then carburized. The part is then copper stripped, finish machined, re-copper plated all over, furnace-hardened, and quenched.
The difficulties and expense of the carburizing process have prompted companies to consider alternative techniques such as induction heating for selective case hardening, but to do so on a larger scale as opposed to the single-tooth method. U.S. Pat. No. 4,675,488, which issued June 23, 1987, to Mucha et al., discloses a variation on the single-tooth process described above, wherein the process involves inductively heating and then quench-hardening a few teeth at a time while the rest of the teeth are cooled for the purpose of preventing drawback of previously hardened teeth (column 1, lines 55-65). While all of the teeth are ultimately induction-hardened, the inductors are extremely complex and expensive. The Mucha et al. patent also mentions the attempt by others for several years to devise a means for induction heating the outer peripheral surfaces of gears by using an encircling inductor so that the gears can be treated by the inductor and then quench-hardened immediately thereafter in order to create the desired case hardening on the outer surface of the gear. The solution suggested by the Mucha et al. patent is to provide two induction heating coils with the workpiece located concentric in the first induction heating coil. This first coil is energized with the first alternating frequency current for a fixed period of time. Once deenergized, the workpiece experiences a time delay period and thereafter the first induction heating coil is reenergized with a second alternating frequency for another fixed period of time, though substantially less than the first period of time with the first alternating frequency. At the end of this second period of time, the workpiece is immediately transferred into the second induction heating coil in a concentric manner and experiences a second delay time. Following this step, the second induction heating coil is energized with a radio frequency current for a third time period and immediately quenching the outer surfaces by quenching liquid sprayed against the surfaces while the workpiece is in the second induction heating coil.
Several years ago, a dual-frequency arrangement for induction heating was described wherein a low-frequency current would be used for preheating the gear teeth and then a high-frequency (radio frequency) current could be used for final heating prior to quench hardening. This dual-frequency arrangement is employed to some extent by the Mucha et al. patent as just described. This dual-frequency concept was also recently described by the present inventors in their article entitled "Induction Gear Hardening by the Dual Frequency Method" which appeared in Heat Treating magazine, Volume 19, No. 6, published in June, 1987, As they explain in their article, the principle of dual-frequency heating employs both high- and low-frequency heat sources. The gear is first heated with a relatively low-frequency source (3-10 KHz), providing the energy required to preheat the mass of the gear teeth. This step is followed immediately by heating with a high-frequency source which will range from 100-300 KHz depending on the gear size and diametral pitch. The high-frequency source will rapidly final heat the entire tooth contour surface to a hardening temperature. The gear is then quenched to a desired hardness and tempered.
Dual-frequency heating is the fastest known way of heating a gear. Heating times range from 0.14 to 2.0 seconds. This compares, for example, with 4-5 minutes required for a laser to scan a gear, tooth by tooth. In dual-frequency heating, the spinning workpiece is preheated while riding on a spindle centering fixture. Then a quick "pulse" achieves optimum final heat. Next the piece indexes into a water-based quench, for a total process time of approximately 30 seconds. Dual frequency is unique among gear-hardening methods in that it allows competing specifications to coexist. That is, for a given case depth requirement and distortion limitation, with conventional hardening methods one requirement tends to consume the other. Because dual-frequency hardening puts only the necessary amount of heat into the part (2-3 times less energy than conventional induction), case depth requirements and distortion specifications can both be met, precisely.
With any induction heating process whether dual- or single-frequency, and regardless of the type of part and its material, the part characteristics dictate the optimum design of both the induction heating coil or coils and the most appropriate machine settings. Only with the properly designed coil and the appropriate machine setting is it possible to achieve the contour and case hardening specifications deemed to be most appropriate from the standpoint of wear and load resistance while still retaining overall part strength, material ductility and part specifications. A gear which is too brittle will fail prematurely, often by a tooth cracking or breaking of the gear blank body.
Traditionally, a fixed coil design has been used for a wide range of different parts and machine settings were made on a "best guess" basis by the induction machine operator. By fixing the coil, one variable is eliminated and the operator attempts to zero in on an acceptable final part by trial and error procedures. The more experienced the operator, hopefully the greater the number and variety of parts he will have experienced and to the extent that he is able to draw on that experience, he may be able to come close to an acceptable part, but only after repeated attempts.
Since this entire approach is not scientific, the best one can hope for is to reach an acceptable part but not an optimum part. This problem is magnified when applying induction heating to irregularly shaped objects such as gears. Heretofore, there has been no attempt to try an derive a set of formulae to precisely determine the most optimal coil specifications and induction machine settings for a given part and which is repeatable, part after part, regardless of the size, shape, material or other characteristics. Instead, gross parameters are selected for the coil based on general part size and then machine settings manipulated until the combination of variables comes close to something that can be accepted.
In order to avoid the uncertainty in coil specifications and machine settings, and to enable induction hardening in a precise and optimum manner, regardless of the type of machine component part or part geometry and features, the present invention provides a machine structure and a method of induction hardening using a series of formulae for establishing coil specifications and machine settings which formulae are based on component part size and features. This process or scientifically calculating the specifications for a unique coil and the machine variables (settings) based on individual part characteristics enables predictable and uniform results for the induction hardening of the part in an orderly and repeatable fashion.
Previously, any calculating which may have been done was rudimentary at best, based only on surface area and depth of penetration. The series of formulae allow the coil and machine variables to be set scientifically rather than by guesswork and the needless trial and error attepts are eliminated while at the same time improving part quality from merely an acceptable or tolerable level to an optimum level.
With the unscientific and haphazard method of prior approaches, there was no control over what variables would be altered by the machine operator and as previously mentioned, altering of one may have an effect on the other such that a suitable combination of variables might never be achieved. With the present invention, although there is some feedback and the possibility if required of adjustment of fine-tuning the settings, only one variable, power, is varied. Further, the feedback and possible need for adjustment is solely to machine component failures or machine parameters going out of tolerance, not because the series of formulae and equations (mathematical algorithm) are imprecise or too loose.